U.S. patent application number 09/817489 was filed with the patent office on 2001-08-02 for method and apparatus for microcellular polypropylene extrusion, and polypropylene articles produced thereby.
Invention is credited to Blizard, Kent, Malavich, William, Tupil, Srinath.
Application Number | 20010010849 09/817489 |
Document ID | / |
Family ID | 21745051 |
Filed Date | 2001-08-02 |
United States Patent
Application |
20010010849 |
Kind Code |
A1 |
Blizard, Kent ; et
al. |
August 2, 2001 |
Method and apparatus for microcellular polypropylene extrusion, and
polypropylene articles produced thereby
Abstract
A polymeric foam article is comprised mainly of homopolymeric
polypropylene, or unimodal polypropylene. The polymeric article can
be microcellular, and can be formed into a drinking straw.
Inventors: |
Blizard, Kent; (Ashland,
MA) ; Tupil, Srinath; (Chelmsford, MA) ;
Malavich, William; (Lowell, MA) |
Correspondence
Address: |
Timothy J. Oyer
Wolf, Greenfield & Sacks, P.C.
600 Atlantic Avenue
Boston
MA
02210-2206
US
|
Family ID: |
21745051 |
Appl. No.: |
09/817489 |
Filed: |
March 26, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09817489 |
Mar 26, 2001 |
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09010289 |
Jan 21, 1998 |
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6231942 |
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Current U.S.
Class: |
428/36.5 ;
239/33; 428/315.7; 428/36.92; 521/142; 521/143; 521/79 |
Current CPC
Class: |
A47G 21/18 20130101;
B29K 2023/12 20130101; B29K 2105/041 20130101; B29C 44/348
20130101; Y10T 428/1397 20150115; B29C 44/50 20130101; C08L 23/12
20130101; B29L 2023/008 20130101; B29C 44/3446 20130101; Y10T
428/1376 20150115; B29C 44/507 20161101; B29K 2105/046 20130101;
Y10T 428/249979 20150401 |
Class at
Publication: |
428/36.5 ;
428/36.92; 428/315.7; 239/33; 521/79; 521/142; 521/143 |
International
Class: |
A47G 021/18; B32B
003/26; B32B 001/08 |
Claims
What is claimed is:
1. An article comprising foamed microcellular polypropylene having
an average cell size less than about 100 microns.
2. An article as in claim 1, the article including at least about
80% by weight polypropylene having a unimodal molecular weight
distribution.
3. An article as in claim 1, the article including at least about
90% by weight polypropylene having a unimodal molecular weight
distribution.
4. An article as in claim 1, the article including at least about
95% by weight polypropylene having a unimodal molecular weight
distribution.
5. An article as in claim 1, consisting essentially of
polypropylene having a unimodal molecular weight distribution.
6. An article as in claim 1, consisting of polypropylene having a
unimodal molecular weight distribution.
7. An article as in claim 1, comprising foamed polymeric material
including at least about 80% by weight homopolymeric polypropylene
of viscosity of at least about 2.5.times.10.sup.3 poise.
8. An article as in claim 1, comprising foamed polymeric material
including at least about 90% by weight homopolymeric polypropylene
of viscosity of at least about 2.5.times.10.sup.3 poise.
9. An article as in claim 1, comprising foamed polymeric material
including at least about 95% by weight homopolymeric polypropylene
of viscosity of at least about 2.5.times.10.sup.3 poise.
10. An article as in claim 1, comprising foamed polymeric material
consisting essentially of homopolymeric polypropylene of viscosity
of at least about 2.5.times.10.sup.3 poise.
11. An article as in claim 1, comprising foamed polymeric material
consisting of homopolymeric polypropylene of viscosity of at least
about 2.5.times.10.sup.3 poise.
12. An article as in claim 1, formed as a drinking straw.
13. An article as in claim 1, having a length-to-diameter ratio of
at least 10, a wall thickness of no more than 1.0 mm.
14. An article as in claim 1, comprising a foamed polymeric tubular
article having a diameter-to-thickness ratio of from about 9:1 to
about 50:1.
15. An article as in claim 1, consisting essentially of
homopolymeric polypropylene having a unimodal molecular weight
distribution and having a viscosity of at least about
2.5.times.10.sup.3 poise.
16. An article comprising foamed polymeric material including at
least about 80% by weight polypropylene having a unimodal molecular
weight distribution.
17. An article as in claim 16, comprising foamed polymeric material
including at least about 90% by weight polypropylene having a
unimodal molecular weight distribution.
18. An article as in claim 16, comprising foamed polymeric material
including at least about 95% by weight polypropylene having a
unimodal molecular weight distribution.
19. An article as in claim 16, comprising foamed polymeric material
consisting essentially of polypropylene having a unimodal molecular
weight distribution.
20. An article as in claim 16, comprising foamed polymeric material
consisting of polypropylene having a unimodal molecular weight
distribution.
21. An article as in claim 16, formed as a drinking straw.
22. An article as in claim 16, having a length-to-diameter ratio of
at least 10, a wall thickness of no more than 1.0 mm.
23. An article as in claim 16, comprising a foamed polymeric
tubular article having a diameter-to-thickness ratio of from about
9:1 to about 50:1.
24. An article comprising foamed polymeric material including at
least about 80% by weight homopolymeric polypropylene of viscosity
of at least about 2.5.times.10.sup.3 poise.
25. An article as in claim 24, comprising the article including at
least about 90% by weight homopolymeric polypropylene of viscosity
of at least about 2.5.times.10.sup.3 poise.
26. An article as in claim 24, comprising the article including at
least about 95% by weight homopolymeric polypropylene of viscosity
of at least about 2.5.times.10.sup.3 poise.
27. An article comprising foamed polymeric material consisting
essentially of homopolymeric polypropylene of viscosity of at least
about 2.5.times.10.sup.3 poise.
28. An article comprising foamed polymeric material consisting of
homopolymeric polypropylene of viscosity of at least about
2.5.times.10.sup.3 poise.
29. An article as in claim 24, comprising foamed polymeric material
including at least about 90% by weight homopolymeric polypropylene
of viscosity of at least about 2.5.times.10.sup.3 poise.
30. An article as in claim 29, having a length-to-diameter ratio of
at least 10, a wall thickness of no more than 1.0 mm.
31. An article as in claim 29, comprising a foamed polymeric
tubular article having a diameter-to-thickness ratio of from about
9:1 to about 50:1.
32. An article comprising polymeric material including at least
about 80% by weight polypropylene having a unimodal molecular
weight distribution and viscosity of at least about
2.5.times.10.sup.3 poise.
33. An article as in claim 32, comprising microcellular
material.
34. An article as in claim 33, consisting essentially of
microcellular polypropylene having a unimodal molecular weight
distribution and a viscosity of at least about 2.5.times.10.sup.3
poise.
35. An article comprising a foam polymeric drinking straw.
36. An article as in claim 35, wherein the foam polymeric drinking
straw comprises microcellular material.
37. An article as in claim 36, wherein the polymeric drinking straw
includes at least about 80% by weight polypropylene having a
unimodal molecular weight distribution.
38. An article as in claim 36, wherein the polymeric drinking straw
includes at least about 80% by weight homopolymeric polypropylene
of viscosity of at least about 2.5.times.10.sup.3 poise.
39. An article as in claim 38, wherein the drinking straw includes
at least about 80% by weight polypropylene having a unimodal
molecular weight distribution.
40. An article as in claim 35, the article comprising a
microcellular polymeric drinking straw consisting essentially of
homopolymeric polypropylene having a unimodal molecular weight
distribution and having a viscosity of at least about
2.5.times.10.sup.3 poise.
41. An article comprising a foamed polymeric tubular article having
a length-to-diameter ratio of at least 10 and a wall thickness of
no more than about 1.0 mm.
42. An article comprising a foamed polymeric tubular article having
a diameter-to-thickness ratio of from about 9:1 to about 50:1.
Description
FIELD OF THE INVENTION
[0001] The present invention relates generally to polymeric foam
processing, and more particularly to a continuous microcellular
polymer extrusion system and method that allows extrusion of
microcellular, standard-grade polypropylene. Polymer foam straws
also are produced.
BACKGROUND OF THE INVENTION
[0002] Foamed polymeric materials are well known, and typically are
produced by introducing a physical blowing agent into a molten
polymeric stream, mixing the blowing agent with the polymer, and
extruding the mixture into the atmosphere while shaping the
mixture. Exposure to atmospheric conditions causes the blowing
agent to gasify, thereby forming cells in the polymer. Under some
conditions the cells can be made to remain isolated, and a
closed-cell foamed material results. Under other, typically more
violent foaming conditions, the cells rupture or become
interconnected and an open-cell material results. As an alternative
to a physical blowing agent, a chemical blowing agent can be used
which undergoes chemical decomposition in the polymer material
causing formation of a gas.
[0003] Foamed polyolefins are known. Of these, polyethylene is
preferred because of ease of foaming control. While foams including
polypropylene components are known, in most cases such foams
include significant proportion of additives that add controlability
to the foaming process.
[0004] U.S. Pat. No. 4,940,736 (Alteeping) describes a foamed
product made by foaming a composition including a major proportion
of a low viscosity polypropylene having a melt viscosity of less
than 2.times.10.sup.3 poise and a minor proportion of a high
viscosity polypropylene having a melt viscosity of greater than
2.5.times.10.sup.3 poise. Alteeping mentions that
previously-proposed procedures for foaming polypropylene had
suffered from severe disadvantages limiting their commercial
application, noting specifically the following: U.S. Pat. No.
4,352,892 (Firma Carl Freudenberg), which discloses foaming a
composition including crystalline polypropylene and a further
component selected from polybutadiene, ethylene vinyl acetate
copolymer, and ethylene-propylene terpolymer rubbers; U.S. Pat. No.
4,442,232 (Firma Carl Freudenberg) which discloses foams comprising
crystalline polypropylene and polybutadiene that are cross-linked;
U.S. Pat. No. 4,298,706 (Karengafuchi Dagaku Koguyo KK) which
discloses foams of compositions comprising of polypropylene and
polybutadiene kneaded together; U.S. Pat. No. 3,846,349 (Sumitomo
Chemical Co.) which describes foam produced from a three-component
mixture of crystalline polypropylene, non-crystalline propylene,
and low density polyethylene; and U.S. Pat. No. 3,607,796
(Grunzweig and Hartmann AG) which describes a process for producing
foam from a composition comprising high and low molecular weight
polypropylene.
[0005] U.S. Pat. No. 5,180,751 (Park) describe polypropylene foam
made of polypropylene resins having a z-average molecular weight
above 1.times.10.sup.6 and a z-average molecular weight/weight
average molecular weight ratio above 3.0. Park states that
unacceptable foam sheets show a unimodal molecular weight
distribution, while resins which yield acceptable foam sheets show
a bimodal molecular weight distribution.
[0006] U.S. Pat. No. 4,832,770 (Nojiri) describes a method of
manufacturing a foamed polypropylene resin from a mixture of 80 to
20 weight percent of a crystalline polypropylene-ethylene block
copolymer containing 20 weight percent or less of ethylene and
having a melt index of two or less and 20 to 80 weight percent of a
crystalline polypropylene-ethylene block or random copolymer
containing 5 weight percent or less of ethylene and having a melt
index of 6 to 20 or a polypropylene homopolymer having a melt index
of 6 to 20.
[0007] One class polymer foams that can offer a variety of
advantageous characteristics such as uniform cell size and
structure, the appearance of solid plastic, etc. are microcellular
foams. U.S. Pat. No. 4,473,665 (Martini-Vvedensky, et al.; Sep. 25,
1984) describes a process for making foamed polymer having cells
less than about 100 microns in diameter. In the technique of
Martini-Vvedensky, et al., a material precursor is saturated with a
blowing agent, the material is placed under high pressure, and the
pressure is rapidly dropped to nucleate the blowing agent and to
allow the formation of cells. The material then is frozen rapidly
to maintain a desired distribution of microcells.
[0008] U.S. Pat. No. 5,158,986 (Cha, et al.; Oct. 27, 1992)
describes formation of microcellular polymeric material using a
supercritical fluid as a blowing agent. In a batch process of Cha,
et al., a plastic article is submerged at pressure in supercritical
fluid for a period of time, and then quickly returned to ambient
conditions creating a solubility change and nucleation. In a
continuous process, a polymeric sheet is extruded, then run through
rollers in a container of supercritical fluid at high pressure, and
then exposed quickly to ambient conditions. In another continuous
process, a supercritical fluid-saturated molten polymeric stream is
established. The stream is rapidly heated, and the resulting
thermodynamic instability (solubility change) creates sites of
nucleation, while the system is maintained under pressure
preventing significant growth of cells. The material then is
injected into a mold cavity where pressure is reduced and cells are
allowed to grow.
[0009] While polymer foams containing polypropylene exist, it would
be advantageous, in terms of added simplicity and reduced cost, to
be able to produce high-quality foams including polypropylene
without the need for significant amounts of foam-controlability
additives or other co-polymerized or blended polymer components. It
is an object of the present invention to provide such articles.
SUMMARY OF THE INVENTION
[0010] The present invention provides methods and systems for
producing polymeric polypropylene foam which can be microcellular
material, and articles produced thereby.
[0011] In one aspect the invention provides extrusion systems.
Specifically, an extruder is provided that has an inlet for
receiving a precursor of a foamed polypropylene material at an
inlet end thereof, and an outlet at an outlet end thereof for
releasing foamed polypropylene material from the extruder. An
enclosed passageway connects the inlet with the outlet. The
passageway is constructed and arranged to contain a product of the
mixture of a blowing agent which can be a supercritical fluid, in
particular supercritical carbon dioxide, with molten polypropylene
material to be foamed within the passageway and to maintain the
product within the passageway. The product can be maintained within
the passageway above the critical temperature and pressure of the
supercritical fluid. A nucleator is associated with the passageway
and is capable of nucleating the product in the passageway in the
absence of auxiliary nucleating agent, although nucleating agent
can be used. An orifice is provided between the inlet and the
outlet and is fluidly connectable to a source of blowing agent
which can be supercritical fluid. The system receives polypropylene
having a unimodal molecular weight distribution or other
polypropylene described below in connection with the articles of
the invention.
[0012] In another aspect the invention provides a method. One
method involves providing a polypropylene material to be foamed,
selected among polypropylene described below with respect to
articles of the invention, and mixing a blowing agent into the
material to create a mixture. A homogeneous single-phase solution
is created from the mixture that has a uniform concentration of
blowing agent distributed therein. The homogeneous single-phase
solution is nucleated, and then essentially immediately thereafter
shaped to create a shaped extrudate. The shaped extrudate can be
released into ambient conditions essentially immediately after
shaping. Nucleation can take place by passing the single-phase
solution through a constriction creating a rapid pressure drop due
to friction.
[0013] In another aspect the invention provides articles. One
article is a foamed microcellular polypropylene article having an
average cell size of less than about 100 microns.
[0014] In another embodiment an article of the invention is a
foamed polymeric article including at least about 80% by weight
polypropylene having a unimodal molecular weight distribution.
[0015] In another embodiment the invention provides an article
including foamed polymeric material including at least about 80% by
weight homopolymeric polypropylene of viscosity of at least about
2.5.times.10.sup.3 poise.
[0016] In another embodiment the invention provides a foam,
polymeric drinking straw.
[0017] The invention also provides a foamed polymeric tubular
article having a length-to-diameter ratio of at least about 10 and
a wall thickness of no more than about 1.0 millimeters.
[0018] The invention also provides a foamed polymeric tubular
article having a diameter-to-thickness ratio of from about 9:1 to
about 50:1.
[0019] Other advantages, novel features, and objects of the
invention will become apparent from the following detailed
description of the invention when considered in conjunction with
the accompanying drawings, which are schematic and which are not
intended to be drawn to scale. In the figures, each identical or
nearly identical component that is illustrated in various figures
is represented by a single numeral. For purposes of clarity, not
every component is labeled in every figure, nor is every component
of each embodiment of the invention shown where illustration is not
necessary to allow those of ordinary skill in the art to understand
the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 is a schematic illustration of a polymeric extrusion
system that can be used in accordance with one embodiment of the
invention;
[0021] FIG. 2 is an alternative polymeric extrusion system that can
be used in accordance with the invention;
[0022] FIG. 3 is a photocopy of an SEM image of microcellular
homopolymeric polypropylene foam of the invention;
[0023] FIG. 4 is a photocopy of an SEM image of microcellular
polypropylene extrudate of the invention;
[0024] FIG. 5 is a photocopy of an SEM image of microcellular
homopolymeric polypropylene of the invention;
[0025] FIG. 6 is a photocopy of an SEM image of talc-filled tubular
microcellular polypropylene of the invention;
[0026] FIG. 7 is a photocopy of an SEM image of talc and color
concentrate-filled tubular microcellular polypropylene of the
invention;
[0027] FIG. 8 is a photocopy of an SEM image of medium-density
microcellular fractional melt flow polypropylene of the invention;
and
[0028] FIG. 9 is a photocopy of an SEM image of medium-density
sheet microcellular polypropylene of the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0029] Commonly-owned, co-pending U.S. patent application Ser. No.
08/777,709 "Method and Apparatus for Microcellular Polymer
Extrusion", filed Dec. 20, 1996 and commonly-owned, co-pending
International patent application serial no. PCT/US97/15088, filed
Aug. 26, 1997 are incorporated herein by reference.
[0030] The various embodiments and aspects of the invention will be
better understood from the following definitions. As used herein,
"nucleation" defines a process by which a homogeneous, single-phase
solution of polymeric material, in which is dissolved molecules of
a species that is a gas under ambient conditions, undergoes
formations of clusters of molecules of the species that define
"nucleation sites", from which cells will grow. That is,
"nucleation" means a change from a homogeneous, single-phase
solution to a mixture in which sites of aggregation of at least
several molecules of blowing agent are formed. Nucleation defines
that transitory state when gas, in solution in a polymer melt,
comes out of solution to form a suspension of bubbles within the
polymer melt. Generally this transition state is forced to occur by
changing the solubility of the polymer melt from a state of
sufficient solubility to contain a certain quantity of gas in
solution to a state of insufficient solubility to contain that same
quantity of gas in solution. Nucleation can be effected by
subjecting the homogeneous, single-phase solution to rapid
thermodynamic instability, such as rapid temperature change, rapid
pressure drop, or both. Rapid pressure drop can be created using a
nucleating pathway, defined below. Rapid temperature change can be
created using a heated portion of an extruder, a hot glycerine
bath, or the like. A "nucleating agent" is a dispersed agent, such
as talc or other filler particles, added to a polymer and able to
promote formation of nucleation sites from a single-phase,
homogeneous solution. Thus "nucleation sites" do not define
locations, within a polymer, at which nucleating agent particles
reside. "Nucleated" refers to a state of a fluid polymeric material
that had contained a single-phase, homogeneous solution including a
dissolved species that is a gas under ambient conditions, following
an event (typically thermodynamic instability) leading to the
formation of nucleation sites. "Non-nucleated" refers to a state
defined by a homogeneous, single-phase solution of polymeric
material and dissolved species that is a gas under ambient
conditions, absent nucleation sites. A "non-nucleated" material can
include nucleating agent such as talc. A "polymeric
material/blowing agent mixture" can be a single-phase,
non-nucleated solution of at least the two, a nucleated solution of
at least the two, or a mixture in which blowing agent cells have
grown. "Essentially closed-cell" microcellular material is meant to
define material that, at a thickness of about 100 microns, contains
no connected cell pathway through the material. "Nucleating
pathway" is meant to define a pathway that forms part of
microcellular polymeric foam extrusion apparatus and in which,
under conditions in which the apparatus is designed to operate
(typically at pressures of from about 1500 to about 30,000 psi
upstream of the nucleator and at flow rates of greater than about
10 pounds polymeric material per hour), the pressure of a
single-phase solution of polymeric material admixed with blowing
agent in the system drops below the saturation pressure for the
particular blowing agent concentration at a rate or rates
facilitating rapid nucleation. A nucleating pathway defines,
optionally with other nucleating pathways, a nucleation or
nucleating region of a device of the invention. "Reinforcing
agent", as used herein, refers to auxiliary, essentially solid
material constructed and arranged to add dimensional stability, or
strength or toughness, to material. Such agents are typified by
fibrous material as described in U.S. Pat. Nos. 4,643,940 and
4,426,470. "Reinforcing agent" does not, by definition, necessarily
include filler or other additives that are not constructed and
arranged to add dimensional stability. Those of ordinary skill in
the art can test an additive to determine whether it is a
reinforcing agent in connection with a particular material.
[0031] In preferred embodiments, the material of the invention is
microcellular material and has average cell size of less than about
50 microns. In some embodiments particularly small cell size is
desired, and in these embodiments material of the invention has
average cell size of less than about 30 microns, more preferably
less than about 20 microns, more preferably less than about 10
microns, and more preferably still less than about 5 microns. The
microcellular material preferably has a maximum cell size of about
100 microns or preferably less than about 75 microns. In
embodiments where particularly small cell size is desired, the
material can have maximum cell size of about 50 microns, more
preferably about 35 microns, and more preferably still about 25
microns. A set of embodiments includes all combinations of these
noted average cell sizes and maximum cell sizes. For example, one
embodiment in this set of embodiments includes microcellular
material having an average cell size of less than about 30 microns
with a maximum cell size of about 50 microns, and as another
example an average cell size of less than about 30 microns with a
maximum cell size of about 35 microns, etc. That is, microcellular
material designed for a variety of purposes can be produced having
a particular combination of average cell size and a maximum cell
size preferable for that purpose. Control of cell size is described
in greater detail below.
[0032] Foam material of the invention has a void volume of at least
about 5%, more preferably at least about 10%, more preferably at
least about 15%, more preferably still at least about 20%, and more
preferably still at least about 30% according to one set of
embodiments. These set of embodiments allow significant reduction
in consumption of polymeric material. In another set of embodiments
the material has a void volume of at least about 50%, more
preferably at least about 60%, more preferably at least about 70%,
and more preferably still at least about 75%. Increasing cell
density while maintaining essentially closed-cell, microcellular
material where that material is desired can be achieved by using
high pressure drop rates as described in international patent
application serial no. PCT/US97/15088, referenced above. Void
volume, in this context, means initial void volume, i.e, typically
void volume immediately after extrusion and cooling to ambient
conditions. That is, formation of foam material at a void volume of
50%, followed by compaction resulting in a void volume of 40%, is
still embraced by the definition of material at 50% void volume in
accordance with the invention.
[0033] The present invention provides systems and techniques for
extrusion of standard-grade or nearly standard-grade polypropylene
foam. Microcellular articles of the invention can be produced
according to a variety of batch or continuous processes, such as
those described in U.S. Pat. No. 5,158,986 of Cha, et al., U.S.
patent application Ser. No. 08/777,709, of Anderson, et al., filed
Dec. 20, 1996 and entitled METHOD AND APPARATUS FOR MICROCELLULAR
POLYMER EXTRUSION, or International Patent Application Serial No.
PCT/US97/15088 of Anderson, et al., filed Aug. 26, 1997, of the
same title, each of which is incorporated herein by reference.
[0034] FIGS. 1 and 2 describe extrusion systems that can be used in
polypropylene foaming according to the present invention. Referring
to FIG. 1, an extrusion system 30 includes a barrel 32 having a
first, upstream end 34 and a second, downstream end 36. Mounted for
rotation within barrel 32 is an extrusion screw 38 operably
connected, at its upstream end, to a drive motor 40. Although not
shown in detail, extrusion screw 38 includes feed, transition, gas
injection, mixing, and metering sections.
[0035] Positioned along extrusion barrel 32, optionally, are
temperature control units 42. Control units 42 can be electrical
heaters, can include passageways for temperature control fluid, or
the like. Units 42 can be used to heat a stream of pelletized or
fluid polymeric material within the extrusion barrel to facilitate
melting, and/or to cool the stream to control viscosity, skin
formation and, in some cases, blowing agent solubility. The
temperature control units can operate differently at different
locations along the barrel, that is, to heat at one or more
locations, and to cool at one or more different locations. Any
number of temperature control units can be provided.
[0036] Extrusion barrel 32 is constructed and arranged to receive a
precursor of polypropylene material. Typically, this involves a
standard hopper 44 for containing pelletized polypropylene to be
fed into the extruder barrel through orifice 46. Although preferred
embodiments do not use chemical blowing agents, when chemical
blowing agents are used they typically are compounded in polymer
pellets introduced into hopper 44.
[0037] Immediately downstream of the downstream end 48 of screw 38
in FIG. 1 is a region 50 which can be a temperature adjustment and
control region, auxiliary mixing region, auxiliary pumping region,
or the like. For example, region 50 can include temperature control
units to adjust the temperature of a fluid polymeric stream prior
to nucleation, as described below. Region 50 can include instead,
or in addition, standard mixing units (not shown), or a
flow-control unit such as a gear pump (not shown). In another
embodiment, region 50 is replaced by a second screw of a tandem
extrusion apparatus, the second screw optionally including a
cooling region.
[0038] Any of a wide variety of blowing agents can be used in
connection with the present invention. Preferably, a physical
blowing agent (a blowing agent that is a gas under ambient
conditions) or mixture of physical blowing agents is used and, in
this case, along barrel 32 of system 30 is a port 54 in fluid
communication with a source 56 of a physical blowing agent.
Physical blowing agents known to those of ordinary skill in the art
such as hydrocarbons, chlorofluorocarbons, nitrogen, carbon
dioxide, and the like can be used in connection with this
embodiment of the invention and, according to a preferred
embodiment, source 56 provides an atmospheric blowing agent, most
preferably carbon dioxide. A pressure and metering device 58
typically is provided between blowing agent source 56 and port 54.
Supercritical fluid blowing agents are especially preferred, in
particular supercritical carbon dioxide. Suitable chemical blowing
agents include those typically relatively low molecular weight
organic compounds that decompose at a critical temperature or
another condition achievable in extrusion and release a gas or
gases such as nitrogen, carbon dioxide, or carbon monoxide.
Examples include azo compounds such as azo dicarbonamide. Where a
chemical blowing agent is used, the blowing agents can be
introduced into systems of a invention by being compounded within
polymer pellets feed into the system, or other techniques available
to those of ordinary skill in the art. Device 58 can be used to
meter the blowing agent so as to control the amount of the blowing
agent in the polymeric stream within the extruder to maintain a
level of blowing agent at a level, according to one set of
embodiments, between about 1% and 15% by weight, preferably between
about 3% and 12% by weight, more preferably between about 5% and
10% by weight, more preferably still between about 7% and 9% by
weight, based on the weight of the polymeric stream and blowing
agent. In other embodiments it is preferred that lower levels of
blowing agent be used. As described in PCT/US97/15088, referenced
above, different levels of blowing agent are desirable under
different conditions and/or for different purposes which can be
selected in accordance with the invention.
[0039] The pressure and metering device can be connected to a
controller (not shown) that also is connected to drive motor 40
and/or a drive mechanism of a gear pump (not shown) to control
metering of blowing agent in relationship to flow of polymeric
material to very precisely control the weight percent blowing agent
in the fluid polymeric mixture.
[0040] Although port 54 can be located at any of a variety of
locations along the extruder barrel, according to a preferred
embodiment it is located just upstream from a mixing section 60 of
the extrusion screw and at a location 62 of the screw where the
screw includes unbroken flights.
[0041] In a preferred embodiment of the blowing agent port system,
two ports on opposing top and bottom sides of the barrel are
provided. In this preferred embodiment, port 54 is located at a
region upstream from mixing section of screw 38 (including
highly-broken flights) at a distance upstream of the mixing section
of no more than about 4 full flights, preferably no more than about
2 full flights, or no more than 1 full flight. Positioned as such,
injected blowing agent is very rapidly and evenly mixed into a
fluid polymeric stream to quickly produce a single-phase solution
of the foamed material precursor and the blowing agent.
[0042] Port 54, in the preferred embodiment is a multi-hole port
including a plurality of orifices connecting the blowing agent
source with the extruder barrel. In preferred embodiments a
plurality of ports 54 are provided about the extruder barrel at
various positions radially and can be in alignment longitudinally
with each other. For example, a plurality of ports 54 can be placed
at the 12 o'clock, 3 o'clock, 6 o'clock, and 9 o'clock positions
about the extruder barrel, each including multiple orifices. In
this manner, where each orifice is considered a blowing agent
orifice, the invention includes extrusion apparatus having at least
about 10, preferably at least about 40, more preferably at least
about 100, more preferably at least about 300, more preferably at
least about 500, and more preferably still at least about 700
blowing agent orifices in fluid communication with the extruder
barrel, fluidly connecting the barrel with a source of blowing
agent.
[0043] Also in preferred embodiments is an arrangement in which the
blowing agent orifice or orifices are positioned along the extruder
barrel at a location where, when a preferred screw is mounted in
the barrel, the orifice or orifices are adjacent full, unbroken
flights. In this manner, as the screw rotates, each flight, passes,
or "wipes" each orifice periodically. This wiping increases rapid
mixing of blowing agent and fluid foamed material precursor by, in
one embodiment, essentially rapidly opening and closing each
orifice by periodically blocking each orifice, when the flight is
large enough relative to the orifice to completely block the
orifice when in alignment therewith. The result is a distribution
of relatively finely-divided, isolated regions of blowing agent in
the fluid polymeric material immediately upon injection and prior
to any mixing. In this arrangement, at a standard screw revolution
speed of about 30 rpm, each orifice is passed by a flight at a rate
of at least about 0.5 passes per second, more preferably at least
about 1 pass per second, more preferably at least about 1.5 passes
per second, and more preferably still at least about 2 passes per
second. In preferred embodiments, orifices are positioned at a
distance of from about 15 to about 30 barrel diameters from the
beginning of the screw (at upstream end 34).
[0044] The described arrangement facilitates a method of the
invention that is practiced according to one set of embodiments.
The method involves introducing, into fluid polymeric material
flowing at a rate of at least about 40 lbs/hr., a blowing agent
that is a gas under ambient conditions and, in a period of less
than about 1 minute, creating a single-phase solution of the
blowing agent fluid in the polymer. The blowing agent fluid is
present in the solution in an amount of at least about 2.5% by
weight based on the weight of the solution in this arrangement. In
preferred embodiments, the rate of flow of the fluid polymeric
material is at least about 60 lbs/hr., more preferably at least
about 80 lbs/hr., and in a particularly preferred embodiment
greater than at least about 100 lbs/hr., and the blowing agent
fluid is added and a single-phase solution formed within one minute
with blowing agent present in the solution in an amount of at least
about 3% by weight, more preferably at least about 5% by weight,
more preferably at least about 7%, and more preferably still at
least about 10% (although, as mentioned, in a another set of
preferred embodiments lower levels of blowing agent are used). In
these arrangements, at least about 2.4 lbs per hour blowing agent,
preferably CO.sub.2, is introduced into the fluid stream and
admixed therein to form a single-phase solution. The rate of
introduction of blowing agent is matched with the rate of flow of
polymer to achieve the optimum blowing agent concentration.
[0045] In the embodiment illustrated in FIG. 1, a system is
provided having a multi-channel nucleator 66, including nucleating
pathways, located substantially upstream of shaping die 68. As used
herein, "nucleating pathway" is meant to define a pathway that
forms part of microcellular polymer foam extrusion apparatus and in
which, under conditions in which the apparatus is designed to
operate (typically at pressures of from about 1500 to about 30,000
psi upstream of the nucleator and at flow rates of greater than
about 10 lbs polymeric material per hour), the pressure of a
single-phase solution of polymeric material admixed with blowing
agent in the system drops below the saturation pressure for the
particular blowing agent concentration at a rate or rates
facilitating nucleation. A nucleating pathway defines, optionally
with other nucleating pathways, a nucleation or nucleating region
of an extruder.
[0046] In one preferred embodiment, nucleator 66 has a polymer
receiving end in fluid communication with the extrusion barrel,
constructed and arranged to receive a fluid, non-nucleated,
single-phase solution of polymeric material and blowing agent
supplied by the barrel. The nucleator includes a nucleated polymer
releasing end in communication with residence chamber 70
constructed and arranged to contain nucleated polymeric material
under conditions controlling cell growth, and a fluid pathway
connecting the receiving end to the releasing end. The arrangement
allows for injecting blowing agent and maintaining the fluid
stream, downstream of injection and upstream of nucleation, under
pressure varying by no more than about 1,000 psi, preferably no
more than about 750 psi, and more preferably still no more than
about 500 psi. The fluid pathway of the nucleator has length and
cross-sectional dimensions that subject the single-phase solution,
as a flowing stream, to conditions of solubility change sufficient
to create sites of nucleation at the microcellular scale in the
absence of auxiliary nucleating agent. "At the microcellular scale"
defines a cell density that, with controlled foaming, can lead to
microcellular material. While nucleating agent can be used in some
embodiments, in other embodiments no new nucleating agent is used.
In either case, the pathway is constructed so as to be able to
create sites of nucleation in the absence of nucleating agent
whether or not nucleating agent is present. In particular, the
fluid pathway has dimensions creating a desired pressure drop rate
through the pathway. In one set of embodiments, the pressure drop
rate is relatively high, and a wide range of pressure drop rates
are achievable. A pressure drop rate can be created, through the
pathway, of at least about 0.1 GPa/sec in molten polymeric material
admixed homogeneously with about 6 wt % CO.sub.2 passing through
the pathway of a rate of about 40 pounds fluid per hour.
Preferably, the dimensions create a pressure drop rate through the
pathway of from about 0.2 GPa/sec to about 1.5 GPa/sec, or from
about 0.2 GPa/sec to about 1 GPa/sec. The nucleator is constructed
and arranged to subject the flowing stream to a pressure drop at a
rate sufficient to create sites of nucleation at a density of at
least about 10.sup.7 sites/cm.sup.3. preferably at least about
10.sup.8 sites/cm.sup.3.
[0047] The arrangement of FIG. 1, or a similar arrangement that
involves a single-channel nucleator located immediately upstream of
shaping in association with a die, is constructed and arranged to
continuously nucleate a fluid stream of single-phase solution of
polymeric material and flowing agent flowing at a rate of at least
20 lbs/hour, preferably at least about 40 lbs/hour, more preferably
at least about 60 lbs/hour, more preferably at least about 80
lbs/hour, and more preferably still at least about 100 lbs/hour. In
FIG. 1 nucleation takes place significantly upstream of shaping. In
the working examples below, nucleation takes place very closely
upstream of final release and shaping. Any arrangement can serve as
a nucleator that subjects a flowing stream of a single-phase
solution of foamed material precursor and blowing agent to a
solubility change sufficient to nucleate the blowing agent. This
solubility change can involve a rapid temperature change, a rapid
pressure change, for example caused by forcing material through an
orifice where the rapid pressure drop takes place due to friction
between the material and the orifice wall, or a combination, and
those of ordinary skill in the art will recognize a variety of
arrangements for achieving nucleation in this manner. A rapid
pressure drop to cause nucleation is preferred. Where a rapid
temperature change is selected to achieve nucleation, temperature
control units can be provided about nucleator 66. Nucleation by
temperature control is described in U.S. Pat. No. 5,158,986 (Cha.,
et al.) incorporated herein by reference. Temperature control units
can be used alone or in combination with a fluid pathway of
nucleator 66 creating a high pressure drop rate in fluid polymeric
material flowing therethrough.
[0048] The described arrangement allows for creation of a
single-phase solution at high flow rates. In particular, the
arrangement allows for establishing the stream of fluid polymeric
material flowing in the extradite at a rate of at least 60 lbs/hour
and introducing CO.sub.2 blowing agent at a rate of at least 1
lb/hour into the stream at an injection location to create a fluid
stream including at least about 2.5% CO.sub.2 by weight.
[0049] While creation of open-cell material is desirable for a
variety of products, closed-cell microcellular polypropylene is
preferred in the present invention. To achieve rapid pressure drop
to create microcellular material, while foaming controllably to
maintain closed-cell material, nucleating should be separated from
shaping by a distance sufficient to achieve this control.
[0050] Also illustrated in FIG. 1 is an optional shaping element 69
downstream of shaping die 68. Shaping element 69 can provide
further control over the thickness or shape of an extruded product
by restricting expansion, further cooling the extradite (via, for
example, fluid cooling channels or other temperature control units
in element 69, not shown), or a combination. Without element 69,
extradite is extruded into ambient conditions upon emergence from
shaping die 68 (restricted only by polymeric extradite downstream
of the exit of the shaping die). With element 69, the extradite
generally emerges from shaping die into conditions of pressure
slightly above ambient.
[0051] With reference to FIG. 1, several arrangements of the
invention are described. In one, polymeric extradite emerges from a
nucleating pathway into ambient conditions and, where multi-channel
nucleation is used, is recombined there. This would involve
elimination of components downstream of nucleator 66. In another
arrangement, only forming element 69 exists downstream of the
nucleator. In another, the system includes nucleator 66, an
enclosure downstream thereof (chamber 70) and a constriction at the
end of the chamber (forming die 68). In still another, the system
includes nucleator 66, chamber 70, forming die 68, and forming
element 69, as illustrated in the complete system of FIG. 1.
Described another way, the invention includes one or more
constrictions constructed and arranged to define nucleating
pathway(s) and one or more constrictions upstream and/or downstream
of the nucleating pathway(s) that each optionally include
temperature control and/or shaping capability. The system produces
extruded article in the shape of a continuous extrusion.
[0052] Referring now to FIG. 2, an alternate extrusion system 71 of
the invention is illustrated schematically, representative of the
system described in the working examples below. System 71 includes
a die 73 similar to die 68 of FIG. 1, but including an exit 75 that
is of dimension creating a nucleating pathway. That is, a
homogeneous, single-phase solution is created by the extruder in
region 50 and, when urged through nucleating pathway 75, the
homogeneous, single-phase solution is nucleated to form a nucleated
fluid polymeric material which then is foamed and shaped optionally
with the assistance of forming element 69.
[0053] Polypropylene foams of the present invention can be blown
with a physical blowing agent such as carbon dioxide and thus, in
preferred embodiments of the invention, the techniques of the
invention do not require the added expense and complication of
formulating a polymeric precursor to include a species that will
react under extrusion conditions to form a blowing agent,
especially the expense and complication of providing a copolymer
component having chemically attached or grafted thereto a chemical
blowing agent. Since foams blown with chemical blowing agents
inherently include residual, unreacted chemical blowing agent after
a final foam product has been produced, as well as chemical
by-products of the reaction that forms a blowing agent, material of
the present invention in this set of embodiments includes residual
chemical blowing agent or reaction by-product of chemical blowing
agent, in an amount less than that inherently found in articles
blown with 0.1% by weight chemical blowing agent or more,
preferably including residual chemical blowing agent or reaction
by-product of chemical blowing agent in an amount less than that
inherently found in articles blown with 0.05% by weight chemical
blowing agent or more. In particularly preferred embodiments, the
material is characterized by being essentially free of residual
chemical blowing agent or free of reaction by-products of chemical
blowing agent. That is, they include less residual chemical blowing
agent or by-product than is inherently found in articles blown with
any chemical blowing agent.
[0054] One advantage of embodiments in which a chemical blowing
agent is not used or used in minute quantities is that
recyclability of product is maximized. Use of a chemical blowing
agent typically reduces the attractiveness of a polymer to
recycling since residual chemical blowing agent and blowing agent
by-products contribute to an overall non-uniform recyclable
material pool.
[0055] In one set of preferred embodiments, a polypropylene foam is
provided that has a unimodal molecular weight distribution. That
is, the added expense and complication of formulating compositions
including copolymers, blends, or the like that have multi-modal
molecular weight distributions is not required. In particular, the
foamed polymeric article of the invention in this set of
embodiments includes at least about 80% by weight polypropylene
having a unimodal molecular weight distribution. In more preferred
embodiments the article includes at least about 90% by weight
polypropylene having a unimodal molecular weight distribution, more
preferably about 95%, and more preferably still the article
consists essentially entirely or consists entirely of unimodal
molecular weight distribution polypropylene. The unimodal or
multi-modal characteristic of a polymer can be readily determined
by those of ordinary skill in the art using, for example, high
temperature gel permeation chromatography (GPC). For example, a
Waters 150 CV GPC chromatograph may be used.
[0056] In another set of preferred embodiments the article is
defined by foamed polymeric material including at least about 80%
by weight homopolymeric polypropylene of viscosity of at least
about 2.5.times.10.sup.3 poise. This embodiment also avoids
complication of significant copolymerization or blending of
auxiliary components. In preferred embodiments, at least about 90%
by weight homopolymeric polypropylene of viscosity of at least
about 2.5.times.10.sup.3 poise is used, more preferably at least
about 95% by weight, and more preferably still the foamed article
consists essentially entirely of or consists entirely of
homopolymeric polypropylene of viscosity of at least about
2.5.times.10.sup.3 poise.
[0057] In accordance with each of these sets of preferred
embodiments, the polypropylene article is preferably at least about
80% free of cross-linking, more preferably at least about 90% free
of cross-linking, or more preferably still essentially entirely
free of cross-linking.
[0058] Preferred embodiments include all sets of combinations of
the above. For example, the article of the invention can include at
least about 80% by weight homopolymeric polypropylene of viscosity
of at least about 2.5.times.10.sup.3 poise that also is unimodal
and is microcellular, or can consist entirely of homopolymeric
polypropylene having a unimodal molecular weight distribution and a
viscosity of at least about 2.5.times.10.sup.3 poise and a maximum
cell size of about 50 microns and an average cell size of about 30
microns and being at least about 90% free of cross-linking,
etc.
[0059] Very thin product, such as sheet, can be made in accordance
with the invention, including tubes and other thin articles.
According to this aspect of the invention, microcellular material,
preferably essentially closed-cell material, of thickness less than
about 4 mm, preferably less than about 3 mm, more preferably less
than about 1 mm is produced. In some embodiments extremely thin
microcellular material is produced, namely material of less than
about 0.5 mm in thickness, more preferably less than about 0.25 mm
in thickness, more preferably still less than about 0.2 mm in
thickness. In some particularly preferred embodiments material on
the order of 0.1 mm in thickness is produced. All of these
embodiments can include essentially closed-cell material.
[0060] Thin product can be formed into a tubular configuration
having, for example, a length-to-diameter ratio of at least 10 and
wall thicknesses as described above. The length-to-diameter ratio
can be at least about 15 in preferred embodiments, more preferably
at least about 20, more preferably at least about 30, and more
preferably still at least about 50. In another embodiment the
article is a tubular article having a diameter-to-thickness ratio
of from about 9:1 to about 50:1, more preferably from about 20:1 to
about 40:1 and more preferably still about 30:1.
[0061] In a particularly preferred embodiment the present invention
finds particular use in the fabrication of drinking straws. Those
of ordinary skill in the art, and indeed the average consumer, will
understand that drinking straws, used especially in connection with
fast-food sales, must have at least the minimum rigidity,
force/deflection value, and columnar strength without collapse, to
puncture a perforated opening in a fast-food beverage cup lid and
to withstand internal vacuum associated with drinking a relatively
viscous beverage such as a milk shake through a straw. It is a
feature of the present invention that foam articles can both be
fabricated at a thickness of a drinking straw, and possesses the
necessary physical properties for drinking straws according to
these commercial requirements. Accordingly, in the one aspect the
invention provides a foam polymeric drinking straw. Use of a foam
polymeric drinking straw both reduces the raw material needed to
produce the straw, and provides the straw with opacity and white
appearance that is desired in the case of many drinking straws. One
way to add opacity to a drinking straw is to add pigment. However,
pigmented polymeric material is less amenable to recycling. The
present invention provides thin, opaque, drinking straws that
include less than about 1% by weight auxiliary opacifer, preferably
less than about 0.05% by weight auxiliary opacifer, and more
preferably still material that is essentially free of auxiliary
opacifer. "Auxiliary opacifer", in the present invention, is meant
to define pigments, dyes, or other species that are designed
specifically to absorb light, or talc or other materials that can
block or diffract light. Those of ordinary skill in the art can
test whether an additive is an opacifer. Microcellular straws of
the invention have the appearance of essentially solid, white,
plastic articles, which offers significant commercial appeal. In
other embodiments, an opacifier can be added.
[0062] Good toughness in the articles of the invention is achieved
without necessity of reinforcing agents. Preferably, the articles
of the invention have less than about 10% reinforcing agent by
weight, more preferably less than about 5% reinforcing agent, more
preferably still less than about 2% reinforcing agent, and in
particularly preferred embodiments the articles of the invention
are essentially free of reinforcing agent. "Reinforcing agent", as
used herein, refers to auxiliary, essentially solid material
constructed and arranged to add dimensional stability, or strength
or toughness, to material. Such agents are typified by fibrous
material as described in U.S. Pat. Nos. 4,643,940 and 4,426,470.
"Reinforcing agent" does not, by definition, include filler,
colorant, or other additives that are not constructed and arranged
to add dimensional stability. Since reinforcing agents are added to
increase dimensional stability, they typically are rod-like in
shape or otherwise shaped to have a ratio, of a maximum dimension
to a minimum dimension (length to diameter in the case of a rod or
fiber) of at least about 3, preferably at least about 5, more
preferably at least about 10.
[0063] The function and advantage of these and other embodiments of
the present invention will be more fully understood from the
examples below. The following examples are intended to illustrate
the benefits of the present invention, but do not exemplify the
full scope of the invention.
EXAMPLE 1
Extrusion of Microcellular Homopolymer Polypropylene Material
Having a Fractional Melt Flow Rate
[0064] A tandem extrusion line (Akron Extruders, Canal Fulton,
Ohio) was arranged including a 2 inch, 32/1 L/D primary extruder
and a 2.5 inch, 34/1 L/D secondary extruder. An injection system
for injection of CO.sub.2 into the primary was placed at a distance
of approximately 20 diameters from the feed section. The injection
system included 4 equally-spaced circumferentially,
radially-positioned ports, each port including 176 orifices, each
orifice of 0.02 inch diameter, for a total of 704 orifices.
[0065] The primary extruder was equipped with a two-stage screw
including conventional first-stage feed, transition, and metering
sections, followed by a multi-flighted (four flights) mixing
section for blowing agent dispersion. The screw was designed for
high-pressure injection of blowing agent with minimized pressure
drop between the first-stage metering section and point of blowing
agent injection. The mixing section included 4 flights unbroken at
the injection ports so that the orifices were wiped (opened and
closed) by the flights. At a screw speed of 80 RPM each orifice was
wiped by a flight at a frequency of 5.3 wipes per second. The
mixing section and injection system allowed for very rapid
establishment of a single-phase solution of blowing agent and
polymeric material.
[0066] The injection system included air-actuated control valve to
precisely meter a mass flow rate of blowing agent at rates from 0.2
to 12 lbs/hr at pressures up to 5500 psi.
[0067] The secondary extruder was equipped with a deep channel,
three-flighted cooling screw with broken flights, which provided
the ability to maintain a pressure profile of microcellular
material precursor, between injection of blowing agent and entrance
to the point of nucleation (the die, in this case) varying by no
more than about 1500 psi, and in most cases considerably less.
[0068] The system included instrumentation allowing measurement of
pressure and temperature of the melt stream at least six locations
throughout the tandem system between a location just prior to the
blowing agent injection ports to the point of entry into the die to
precisely monitor material conditions. Along the screw, melt
temperature was measured with infrared equipment to avoid
disruption of the melt stream.
[0069] PP pellets were gravity-fed from a hopper into the extrusion
system. The grade used was a standard homopolymer resin (Montell
6823), having a nominal melt flow index of 0.5 g/10 min. Primary
screw speed was 40 RPM, giving a total output of approximately 36
lbs/hr of material. Secondary screw speed was 13 RPM. Barrel
temperatures of the secondary extruder were set to maintain a melt
temperature of 380.degree. F. measured at the end of the secondary
extruder. CO.sub.2 blowing agent was injected at a rate of 1.8
lbs/hr resulting in 5.0% blowing agent in the melt. Pressure
profile between the injection ports and the inlet of the die was
maintained between 2350 and 2780 psi. The die placed at the end of
the secondary extruder was a circular orifice of constant cross
section with a diameter of 0.080 inches and a length of 1.2 inches.
The pressure drop rate across the die was approximately 1.1
GPa/s.
[0070] FIG. 3 is a photocopy of an SEM image of the cross section
of the extrudate, showing uniform, spherical, relatively closed
cells, the majority of which were from 20 to 50 microns in
diameter. Material density was approximately 0.35 g/cm.sup.3 (19.3
lbs/ft.sup.3), and cell density was approximately
3.7.times.10.sup.7 cells/cm.sup.3.
EXAMPLE 2
Extrusion of Microcellular Homopolymer Polypropylene Material
Having Talc Filler
[0071] System and parameters were used as in Example 1. PP pellets
were gravity-fed from a hopper into the extrusion system. The grade
used was a standard talc-filled homopolymer resin (Montell Astryn
65F4-4). The base resin had a nominal melt flow index of 4 g/10
min., and was filled with 40% by weight of talc. Primary screw
speed was 60 RPM, giving a total output of approximately 63 lbs/hr
of material. Secondary screw speed was 24 RPM. Barrel temperatures
of the secondary extruder were set to maintain a melt temperature
of 326.degree. F. measured at the end of the secondary extruder.
CO.sub.2 blowing agent was injected at a rate of 2.0 lbs/hr
resulting in 5.3% blowing agent in the melt. Pressure profile
between the injection ports and the inlet of the die was maintained
between 2560 psi and 3250 psi. The die placed at the end of the
secondary extruder was a circular orifice of constant cross section
with a diameter of 0.080 inches and a length of 1.2 inches. The
pressure drop rate across the die was 1.7 GPa/s.
[0072] FIG. 4 is a photocopy of an SEM image of the cross section
of the extrudate, showing uniform, partially closed cells of an
average of about 10 microns diameter, with a maximum size of about
30 microns diameter. Material density was approximately 0.61 g/cm3
(38 lbs/ft3), and cell density was approximately 1.0.times.109
cells/cm3.
EXAMPLE 3
Extrusion of Microcellular Homopolymer Polypropylene Low Density
Sheet
[0073] System and parameters were used as in Example 1. PP pellets
were gravity-fed from a hopper into the extrusion system. The grade
used was a standard homopolymer resin (Montell 6823), having a
nominal melt flow index of 0.5 g/10 min. Primary screw speed was 80
RPM, giving a total output of approximately 65 lbs/hr of material.
Secondary screw speed was 20 RPM. Barrel temperatures of the
secondary extruder were set to maintain a melt temperature of
328.degree. F. measured at the end of the secondary extruder.
CO.sub.2 blowing agent was injected at a rate of 5.2 lbs/hr
resulting in 8.0% blowing agent in the melt. Pressure profile
between the injection ports and the inlet of the die was maintained
between 2630 psi and 3880 psi. The system included, at the end of
the secondary extruder, a die adapter and a cylindrical annular die
with an exit gap of 0.053 inches with a diameter of 1.25 inches
diverging from a nucleation gap of 0.022 thickness, 0.563 inch
length, and diameter of 0.853 inch. The die adapter was equipped
with taps for measurement of melt temperature and pressure just
prior to entry into the die. The pressure drop rate across the die
was 0.4 GPa/s.
[0074] FIG. 5 is a photocopy of an SEM image of the cross section
of the extrudate, showing nominally hexagonal-shaped thin wall
cells at least partially closed in structure. Average cell size was
about 70 microns, with maximum size of 100 microns diameter.
Material density was measured to be 0.13 g/cm.sup.3 (8.1
lbs/ft.sup.3), and cell density was approximately
1.7.times.10.sup.7 cells/cm.sup.3.
EXAMPLE 4
Microcellular Polypropylene Talc Filled Tubular Product
[0075] An NRM (Pawcatuck, Conn.) 2.5 inch 44/1 L/D long single
extrusion line was equipped with an injection system for injection
of CO.sub.2 placed at a distance of approximately 25 diameters from
the feed section. The injection system included 4 equally-spaced
circumferentially, radially-positioned ports, each port including
417 orifices, each orifice of 0.02 inch diameter, for a total of
1668 orifices.
[0076] The extruder was equipped with a two-stage screw including
conventional first-stage feed, barrier flight transition, and
metering sections, followed by a multi-flighted (six flights)
mixing section for blowing agent dispersion. The screw was designed
for high-pressure injection of blowing agent with minimized
pressure drop between the first-stage metering section and point of
blowing agent injection. The second stage of the screw included a
mixing section having 6 flights unbroken at the injection ports so
that the orifices were wiped (opened and closed) by the flights. At
a screw speed of 80 RPM each orifice was wiped by a flight at a
frequency of 8 wipes per second. The mixing section and injection
system allowed for very rapid establishment of a single-phase
solution of blowing agent and polymeric material. The injection
system included an air-actuated control valve to precisely meter a
mass flow rate of blowing agent at rates from 0.2 to 50 lbs/hr at
pressures up to 5500 psi.
[0077] The second stage of the screw was also equipped with a deep
channel, three-flighted cooling section with broken flights, which
provided the ability to cool the polymer melt stream.
[0078] The system included, at the end of the extruder, a die
adapter and a cylindrical annular die with a gap of 0.34 inch,
inner diameter of 0.4 inch, and land length of 2 inches. The die
adapter was equipped with taps for measurement of melt temperature
and pressure just prior to entry into the die.
[0079] The system included instrumentation allowing measurement of
pressure and temperature of the melt stream at least 7 locations
throughout the system between a location just prior to the blowing
agent injection ports to the point of entry into the die to
precisely monitor material conditions. Along the screw, melt
temperature was measured with infrared equipment to avoid
disruption of the melt stream.
[0080] A standard homopolymer polypropylene resin (Solvay HB 1301),
having a nominal melt flow index of 5 g/10 min., was used as the
base resin. Talc concentrate consisting of pellets having 40
percent by weight of talc dispersed in a homopolymer polypropylene
matrix were blended with the HB 1301 using a loss-in-weight type
blending system to produce a mixture containing 5% by weight of
talc. This mixture was then gravity fed from a hopper into the
extrusion system. Primary screw speed was 50 RPM, giving a total
output of approximately 54 lbs/hr of material. Barrel temperatures
were set to maintain a melt temperature of 422.degree. F. measured
at the end of the extruder. CO.sub.2 blowing agent was injected at
a rate of 0.3 lbs/hr resulting in 0.5 5% blowing agent in the melt.
A die adapter was attached to the discharge of the extruder,
connecting to a cylindrical annular die having a gap of 0.025
inches with an outer diameter of 0.118 inches and a land length of
0.15 inches. Pressure profile between the injection ports and the
inlet of the die was maintained between 2430 and 3540 psi. The
pressure drop rate across the die was 11.2 GPa/s.
[0081] FIG. 6 is a photocopy of an SEM image of the cross section
of the extrudate, showing generally spherical cells approximately
50 microns in diameter dispersed throughout the cross section of
the tube wall. Wall thickness of the product was approximately
0.008 inch (0.21 mm). Product outer diameter was about 0.26 inch
(6.60 mm). Material density was approximately 0.51 g/cm.sup.3 (32
lbs/ft.sup.3), and cell density was approximately
2.0.times.10.sup.7 cells/cm.sup.3.
EXAMPLE 5
Microcellular Polypropylene Tubular Product with Talc and Color
Concentrate
[0082] System and parameters were used as in Example 4. A resin
formulation having the same base PP resin grade as Example 4, but
with 4% of TiO.sub.2 pellet color concentrate and 3% talc was
blended and gravity fed from a hopper into the extrusion system.
Primary screw speed was 50 RPM, giving a total output of
approximately 54 lbs/hr of material. Barrel temperatures were set
to maintain a melt temperature of 404.degree. F. measured at the
end of the extruder. CO.sub.2 blowing agent was injected at a rate
of 0.3 lbs/hr resulting in 0.55% blowing agent in the melt. A die
adapter was attached to the discharge of the extruder, connecting
to a cylindrical annular die having a gap of 0.025 inches with an
outer diameter of 0.18 inches and a land length of 0.15 inches.
Pressure profile between the injection ports and the inlet of the
die was maintained between 2710 and 3950 psi. The pressure drop
rate across the die was 12.6 GPa/s.
[0083] FIG. 7 is a photocopy of an SEM image of the cross section
of the extrudate, showing generally spherical cells approximately
35 microns in diameter dispersed throughout the cross section of
the tube wall. Wall thickness of the product was approximately
0.007 inch (0.18 mm). Product outer diameter was about 0.26 inch
(6.60 mm). Material density was approximately 0.57 g/cm.sup.3 (36
lbs/ft.sup.3), and cell density was approximately
5.2.times.10.sup.6 cells/cm.sup.3.
EXAMPLE 6
Microcellular Fractional Melt Flow Polypropylene Medium Density
Sheet
[0084] System and parameters were used as in Example 1, but with a
2.5 inch primary extruder and a 3 inch secondary extruder. PP
pellets were gravity-fed from a hopper into the extrusion system.
The grade used was a standard homopolymer resin (Montell 6823),
having a nominal melt flow index of 0.5 g/10 min. Primary screw
speed was 90 RPM, giving a total output of approximately 84 lbs/hr
of material. Secondary screw speed was 5 RPM. Barrel temperatures
of the secondary extruder were set to maintain a melt temperature
of 386.degree. F. measured at the end of the secondary extruder.
CO.sub.2 blowing agent was injected at a rate of 4.0 lbs/hr
resulting in 4.8% blowing agent in the melt. A die adapter at the
discharge of the secondary extruder was connected to a flat sheet
T-type die having a die exit of 4.5 inches width and gap of 0.034
inch. A separate nucleator of constantly decreasing gap to an exit
dimension of 0.015 inch was positioned within 0.5 inches of the die
exit. The die had both melt and pressure indicators. Pressure
profile between the injeciton ports and the inlet of the die was
maintainted between 2120 and 3490 psi. The overall pressure drop
rate across the die lips was 0.07 GPa/s.
[0085] FIG. 8 is a photocopy of an SEM image of the cross section
of the extrudate, showing a uniform dispersion of cells having an
average diameter of about 25 microns. Material density was
approximately 0.63 g/cm.sup.3 (39 lbs/ft.sup.3, and cell density
was approximately 2.7.times.10.sup.7 cells/cm.sup.3.
EXAMPLE 7
Microcellular Polypropylene Medium Density Sheet
[0086] The extrusion system was identical to that of Example 6,
with the exception of the die lip dimensions. PP pellets were
gravity-fed from a hopper into the extrusion system. The grade used
was a standard homopolymer resin (Solvay HB 3052), having a nominal
melt flow index of 1.5 g/10 min. Primary screw speed was 50 RPM,
giving a total output of approximately 100 lbs/hr of material.
Secondary screw speed was 13 RPM. Barrel temperatures of the
secondary extruder were set to maintain a melt temperature of
379.degree. F. measured at the end of the secondary extruder.
CO.sub.2 blowing agent was injected at a rate of 5.5 lbs/hr
resulting in 5.5% blowing agent in the melt. The die exit width was
10 inches, with a constant 5.degree. included angle tapered exit of
0.5 inch length having and exit gap of 0.007 inch. Pressure profile
between the injection ports and the inlet of the die was
maintainted between 1960 and 2210 psi. The overall pressure drop
rate across the die lips was 0.06 GPa/s.
[0087] FIG. 9 is a photocopy of an SEM image of the cross section
of the extrudate, showing small unifrom, homogeneously distributed
cells of average diameter of approximately 10 microns. Material
density was approximately 0.57 g/cm.sup.3 (36 lbs/ft.sup.3), and
cell density was approximately 5.8.times.10.sup.8
cells/cm.sup.3.
[0088] Those skilled in the art would readily appreciate that all
parameters listed herein are meant to be exemplary and that actual
parameters will depend upon the specific application for which the
methods and apparatus of the present invention are used. It is,
therefore, to be understood that the foregoing embodiments are
presented by way of example only and that, within the scope of the
appended claims and equivalents thereto, the invention may be
practiced otherwise than as specifically described.
* * * * *